Lecture Data communications and networks: Chapter 20 - Forouzan 

Chapter 20 - Network layer: internet protocol. This chapter is devoted to the main protocol at the network layer that supervises and controls the delivery of packets from the source to destination. This protocol is called the Internet Protocol or IP.

20-1 INTERNETWORKING

In this section, we discuss internetworking, connecting  networks  together  to  make  an  internetwork  or  an  internet. 

Need for Network Layer

Internet as a Datagram Network

Internet as a Connectionless Network

Topics discussed in this section:

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Communication at the network layer in

the Internet is connectionless.

Note

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The precedence subfield was part of

version 4, but never used.

Note

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Example 20.1

In an IPv4 packet, the value of HLEN is 1000 in binary.  How  many  bytes  of  options  are  being  carried  by  this  packet?

Solution

The HLEN value is 8, which means the total number of bytes in the header is 8 × 4, or 32 bytes The first 20 bytes are the base header, the next 12 bytes are the options.

Example 20.2

Example 20.3

An IPv4 packet has arrived with the first few hexadecimal  digits as shown.

time-to-is IGMP.

Example 20.4

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A packet has arrived with an M bit value of 0. Is this the  first fragment, the last fragment, or a middle fragment? 

Do we know if the packet was fragmented?

Solution

If the M bit is 0, it means that there are no more fragments; the fragment is the last one However, we cannot say if the original packet was fragmented or not A non-fragmented packet is considered the last fragment.

Example 20.5

A packet has arrived with an M bit value of 1. Is this the  first fragment, the last fragment, or a middle fragment? 

Do we know if the packet was fragmented?

Solution

If the M bit is 1, it means that there is at least one more fragment This fragment can be the first one or a middle one, but not the last one We don’t know if it is the first one or a middle one; we need more information (the value of the fragmentation offset).

Example 20.6

A  packet  has  arrived  with  an  M  bit  value  of  1  and  a  fragmentation offset value of 0. Is this the first fragment,  the last fragment, or a middle fragment?

Solution

Because the M bit is 1, it is either the first fragment or a middle one Because the offset value is 0, it is the first fragment.

Example 20.7

A  packet  has  arrived  in  which  the  offset  value  is  100.  What  is  the  number  of  the  first  byte?  Do  we  know  the  number of the last byte?

Solution

To find the number of the first byte, we multiply the offset  value by 8. This means that the first byte number is 800. 

We cannot determine the number of the last byte unless 

we know the length.

Example 20.8

A packet has arrived in which the offset value is 100, the  value of HLEN is 5, and the value of the total length field 

is 100. What are the numbers of the first byte and the last  byte?

Solution

The first byte number is 100 × 8 = 800 The total length is

100 bytes, and the header length is 20 bytes (5 × 4), which means that there are 80 bytes in this datagram If the first byte number is 800, the last byte number must be 879.

Example 20.9

Figure  20.13  shows  an  example  of  a  checksum  calculation  for  an  IPv4  header  without  options.  The  header is divided into 16­bit sections. All the sections are  added  and  the  sum  is  complemented.  The  result  is  inserted in the checksum field.

Example 20.10

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20-3 IPv6

The  network  layer  protocol  in  the  TCP/IP  protocol  suite is currently IPv4. Although IPv4 is well designed,  data communication has evolved since the inception of  IPv4  in  the  1970s.  IPv4  has  some  deficiencies  that  make it unsuitable for the fast­growing Internet.

Advantages

Packet Format

Extension Headers

Topics discussed in this section:

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20-4 TRANSITION FROM IPv4 TO IPv6

Because  of  the  huge  number  of  systems  on  the  Internet,  the  transition  from  IPv4  to  IPv6  cannot  happen  suddenly.  It  takes  a  considerable  amount  of  time before every system in the Internet can move from  IPv4 to IPv6. The transition must be smooth to prevent  any problems between IPv4 and IPv6 systems. 

Dual Stack

Tunneling

Header Translation

Topics discussed in this section:

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